Copying Action From One armature to Another armature
An object can have only one active action, this is the action that receives new keyframes when you keyframe a value. The same action can be assigned to multiple objects. The dopesheet has several modes which includes action editor which can be used to edit or create, delete and assign actions to an object. The NLA editor can be used to have multiple actions affect the same object and also has the ability to repeat, ret-ime and offset an action over time.

An action is a collection of fcurves that contains keyframed values for an objects property/s. Each fcurve contains a data path that specifies the property that the keyframes effect. While the dopesheet can adjust the timing, the graph editor is used to edit other details of fcurves, including the interpolation mode of the curve.

For an action to be usable on multiple objects, they need to have the same data path for property names, for things like location and rotation that simply applies to every object. For an armature the "data path" recorded for each keyframe includes the bone name, so to use an action on multiple armatures they need to have matching bone names. The keyframes in an action will animate the bones with matching names the same in each armature.